“Environmental extremists believe criminal actions, to include physical and economic damage inflicted on the infrastructure, individuals, and businesses associated with the oil and natural gas industry, are justified and necessary to end perceived harm to the environment,” says a memo dated January 23, 2013, titled “Threats to Keystone XL Pipeline Projects Within Houston Domain.”

“Many of these extremists believe the debates over pollution, protection of wildlife, safety, and property rights have been overshadowed by the promise of jobs and cheaper oil prices,” the memo added. “The Keystone pipeline, as part of the oil and natural gas industry, is vital to the security and economy of the United States.”

The Keystone XL pipeline would transport tar-sands oil from the Canadian province of Alberta to refineries in Houston and the Texas Gulf Coast area. Environmental groups have fiercely opposed it. James Hansen, a climate scientist with the National Aeronautics and Space Administration, has said that if it’s built, “it will be game over for the climate.” But the oil industry, Republicans, and construction unions have supported it.

President Barack Obama has not yet decided to approve construction.

Internal documents obtained under the Freedom of Information Act also reveal that the surveillance violated the bureau’s guidelines for investigating political groups. The agents spied on environmental activists from November 2012 through June 2014 without getting approval from both the special agent in charge and the head lawyer in the field office. “FBI approval levels required by internal policy were not initially obtained,” the bureau responded in a statement, but once the operation was discovered—apparently in August 2013—“corrective action was taken.”

The investigation was aimed at Tar Sands Blockade, a direct-action group which began organizing in 2012 in the east Houston neighborhood of Manchester, the site of a large refinery owned by the Valero Energy Corporation. The spying apparently began “after a high-level strategy meeting between the agency and TransCanada, the company building the pipeline,” the two publications said. According to the more than 80 pages of documents:

…the FBI collated inside knowledge about forthcoming protests, documented the identities of individuals photographing oil-related infrastructure, scrutinized police intelligence, and cultivated at least one informant. It is unclear whether the source or sources were protesters-turned-informants, private investigators or hackers. One source is referred to in the documents as having had “good access and a history of reliable reporting.” At one point, the FBI’s Houston office said it would share with TransCanada “any pertinent intelligence regarding any threats” to the company in advance of a forthcoming protest.

Tar Sands Blockade organizer Ron Seifert told the Guardian that dozens of activists affiliated with the group had been arrested in Texas for civil-disobedience charges such as trespassing, but none of them were accused of violent crime or property destruction.

The FBI said it was compelled to “take the initiative to secure and protect activities and entities which may be targeted for terrorism or espionage.” Former FBI agent Mike German, now a fellow at the Brennan Center for Justice in New York, told the Guardian that the documents indicated the investigation was what the FBI calls an “assessment.”

“It is clearly troubling that these documents suggest the FBI interprets its national security mandate as protecting private industry from political criticism,” he said.

An “assessment” is essentially a fishing expedition permitted by regulations established after the 9/11 attacks. Those rules allow agents to investigate individuals or groups without any evidence they are breaking the law, if a plausible connection to “terrorism” can be claimed.

The Tar Sands Blockade investigation was closed after the Houston division failed to find significant evidence of “extremist activity,” according to the documents—but the bureau still had a file designated for future information “regarding the Keystone XL pipeline.” The Houston investigation may not have been the only FBI probe of anti-Keystone activists, the documents appear to hint. It certainly wasn’t the bureau’s only surveillance of environmental activists. Over the last two years, the bureau has contacted and tried to question several people in Oregon, Washington, and Idaho affiliated with the group Rising Tide, which has blockaded trucks transporting drilling gear from the northwestern U.S. to the Alberta tar-sands fields.